The present invention relates generally to the arts of data communications and telephony, and more particularly, to a central office (CO) filter system (and associated methodology) for connection between a CO telephone switch and a telephone connection (e.g., subscriber loop) that can communicate voice and digital data signals, for the purpose of suppressing transients and harmonics on the connection caused by a telephone ring signal generated by the switch. The suppression advantageously minimizes adverse effects on the digital data channel on the connection, particularly voice signals that are communicated along the one or more digital data channels.
Recently, in the art of telephony, 2-wire copper telephone connections, particularly, those that make up subscriber loops between a telephone central office (CO) and customer premises (CP), have been utilized for concurrently communicating more than one signal, including digital data, digital voice, and analog voice (i.e., plain old telephone service (POTS)) signals. For example, a high speed digital subscriber line (DSL) channel (e.g., ADSL, SDSL, RADSL, VADSL, etc.) and an analog plain old telephone system (POTS) channel can be established concurrently over a single physical 2-wire connection. The signals are typically separated in frequency. The POTS channel usually exhibits a frequency spectrum of about 0 KHz to about 4 KHz, whereas the DSL channel exhibits a frequency spectrum of about 20 KHz to about 500 KHz with a bit rate of about 1.5 Mbits/sec to 52 Mbits/sec.
A new DSL technology known as xe2x80x9cMVL(trademark),xe2x80x9d or xe2x80x9cmultiple virtual lines,xe2x80x9d which has recently been created by and is commercially available from Paradyne Corporation, U.S.A., an innovator in DSL technology, enables simultaneous transmission of digital data and analog voice signals over a single 2-wire pair connection. This technology is described in commonly assigned application entitled, xe2x80x9cAPPARATUS AND METHOD FOR COMMUNICATING VOICE AND DATA BETWEEN A CUSTOMER PREMISES AND A CENTRAL OFFICE,xe2x80x9d filed Nov. 3, 1997, and accorded U.S. patent application Ser. No. 08/862,796.
Another new DSL technology known as xe2x80x9cTripleplay(trademark),xe2x80x9d which has also recently been created by and is commercially available from Paradyne Corporation, U.S.A., permits simultaneous communication of a digital data signal, an analog voice signal, and up to two digital voice signals, over a single 2-wire pair connection.
When multiple channels are communicated across a connection, historically, a POTS splitter has been utilized to decouple the channels. A POTS splitter is typically situated at the CO as well as at the CP. A POTS splitter is typically a passive or active one-to-three port device. It often includes a low pass filter to minimize high frequency transients produced by on-hook/off-hook transitions of telephone equipment, so as to prevent tainting or slowing of the high speed data on the DSL channel(s). Also, it is usually configured to provide a high impedance to the telephone line in the DSL frequency band in order to prevent DSL power from being imparted on the POTS communications device that is connected to the line.
The specification of POTS splitters has been the subject of several industry standards bodies. For example, see American National Standards Institute, ANSI T1.413-1995, Sections 8 and 10, regarding ADSL/POTS splitters. Moreover, as an example of a possible implementation of a POTS splitter, see J. Cook, P. Sheppard, xe2x80x9cADSL and VADSL Splitter Design and Telephony Performance,xe2x80x9d IEEE Journal on Selected Areas in Communications, December 1995.
Although transmission of both digital data and analog voice signals along a subscriber loop between a telephone switch at a CO and a CP is possible, it is not free of problems. A significant problem arises when the telephone switch initiates a ring signal along the subscriber loop. The ring signal is a high amplitude periodic wave form that is communicated for two second intervals, with each ring separated by four second time periods. The ring signal is specified in FCC Part 68. Generally, the ring signal is an alternating current (AC) signal having a root-mean-squared (rms) voltage of up to about 150 volts. The ring signal was designed to be very large in order to properly actuate electromechancial ringers that were present in earlier versions of telephones. Unfortunately, the application of the baseband ringing current imposes undesirable noise upon the one or more other digital data channels on the subscriber loop, particularly the digital voice channel(s), for example, that provided in Paradyne""s Tripleplay technology, in the form of transients and harmonics. For adequate performance, the digital voice channel(s) need to experience low bit errors.
Thus, a heretofore unaddressed need exists in the industry for a way to prevent or substantially minimize the adverse effects of a ring signal upon the one or more digital data channels that are communicated along the subscriber loop with the analog voice channel.
The present invention provides a central office (CO) filter system (and associated methodology) for connection between a CO telephone switch and a telephone connection (e.g., subscriber loop) that can communicate voice and digital data signals, for the purpose of suppressing transients and harmonics on the connection caused by a telephone ring signal generated by the telephone switch. The suppression advantageously minimizes adverse affects on the digital data channel on the connection, particularly voice signals that are communicated along the one or more digital data channels between the CO and customer premises (CP).
In architecture, the CO filter system includes a first filter designed to implement a first low pass filter function to attenuate frequencies higher than a ring signal so that transients and harmonics produced from the ring signal are suppressed, a second filter designed to implement a second low pass filter function to attenuate frequencies higher than a voice channel, and a current detector designed to detect when a telephone is off-hook. The current detector implements the first filter when the telephone is on-hook, particularly, when the ring signal occurs, and implements the second filter when the telephone is off-hook.
In another sense, the present invention may be viewed as providing a method for suppressing transients and harmonics on a telephone connection that communicates analog and digital data channels and that is caused by a telephone ring signal generated by a telephone switch in order to minimize adverse effects on a digital data channel on the connection. In this regard, the method can be broadly viewed as comprising the following steps: (a) implementing a first transfer function during a ring signal, the ring signal occurring when a telephone connected to said telephone connection is on-hook, the first transfer function configured to attenuate frequencies higher than said ring signal in order to suppress transients and harmonics produced by the ring signal; and (b) implementing a second transfer function when the telephone is off-hook, the second transfer function configured to permit passage of an analog voice channel, while attenuating frequencies higher than the analog voice channel.